The soybean, Glycine max (L.) Merrill, was domesticated by the farmers in the eastern half of northern China during the Shang dynasty (ca. 1700-1100 B.C.), and first introduced to North America in 1765 by Samuel Bowen, a seaman employed by the East India Company, from China via London to Savannah (in the colony of Georgia). He manufactured soy sauce and exported it to London. In 1851, soybeans were introduced into the midwestern U.S. by Dr. Benjamin F. Edwards, who gave them to J. H. Lea, of Alton, Ill., who planted them in his garden. By 1854, soybeans had been disseminated throughout the United States.
World oilseed production for 1987-88 was estimated at a record 202 million tons, with soybeans accounting for half the total crop. The United States produced 51.8 million tons of soybeans, representing over 51% of world production. Brazil had a record harvest of 18.5 million tons, and the People's Republic of China harvested 11.8 million tons. The fourth highest producer was Argentina with 8.5 million tons.
In 1987, the worldwide loss to soybean diseases was estimated at 10.3 million metric tons. Losses in the United States were estimated at 50-60 million dollars in 1986 (estimates vary, depending upon market price). More than 100 pathogens are known to affect soybeans; about 35 are important economically. Some of the most important are those that cause damage to the roots and crowns of soybeans. One of the most important root and crown rot diseases are those caused by Rhizoctonia solani. Rhizoctonia diseases, including pre- and post-emergence damping-off, root and stem decay, and leaf and bud blight, have been reported in all soybean-growing areas of the world. Pre- and post-emergence damping-off and root decay can reduce stands by as much as 50% and losses of up to 40% have been recorded in Brazil and the U.S. The causal fungus has a wide host range, which includes field crops, vegetables, ornamentals, and fruit crops.
Pre-emergence blight caused by R. solani occurs immediately after the seedling emerges from the seed. The sprouted seed is killed and decayed by the causal fungus. Damping-off can occur a few days after emergence. Reddish-brown lesions appear at the base of the seedling stem and on roots just below the soil line. These may enlarge into sunken lesions, which may girdle the stem. Lesions and cankers may so weaken the stem that plants break off in mid-season or die. Decay may continue intermittently throughout the growing season, with continuing death of plants. Infected plants that survive show some yield reduction.
R. solani is primarily a soil inhabitant and has excellent saprophytic ability. The fungus can overseason in the absence of the host, and colonizes all types of plant debris. Growth in soil depends on nutrient supply, soil moisture, temperature, pH, and competition from other soil microorganisms. The population of the fungus generally is distributed mainly in the upper 10 cm (centimeters) of the soil, decreasing with depth to about 50 cm below the surface. When environmental conditions are optimal for the fungus, disease severity is directly related to inoculum potential. An inoculum density of 100 ug of mycelia/gram of soil can cause severe disease in soybeans. The fungus produces pectolytic and proteolytic enzymes which play a role in disease pathology.
There is no natural resistance to R. solani in soybeans or any other crop. An integrated disease management program has been used heretofore against this fungus which includes agronomic practices as well as fungicide seed protectants. The seed protectants which may be used alone or in combination with other fungicides for this purpose are: quintozene (pentachloronitrobenzene, Terraclor) or carboxin (DCMO, Vitavax) . Both, but particularly quintozene, cause delayed emergence and growth of seedlings. An integrated disease management program is required, which would include the use of a suppressive biological control agent. The present invention details the use of such an agent, and in particular, the use of particular Bacillus megaterium strains as biological control agents for R. solani.
The agricultural use of B. megaterium has been previously reported for disease control in rice and cotton. Inhibition of Drechslera oryzae, which causes brown spot disease in rice, by B. megaterium, and subsequent control of the disease was reported with regular spraying of a bacterial suspension. In field studies, spraying with a suspension of B. megaterium reduced rice disease incidence and resulted in better growth and higher rice yields [Islam, K. Z., et al. (1985) Z. Pflkrankh. Pflschutz (Journal of Plant Diseases and Protection) 92:241-246]. The use of a suspension of B. megaterium reduced the number of cotton plants killed by Phymatotrichum omnivorum, cause of cotton root rot, by 25% with a resulting yield increase in lint of 24% over the control [Cook, C. G. , et al. (1987), Proc. Beltwide Cotton Production--Mechanization Research Conf. Memphis, p. 43-45.]
Antibiotic production from B. megaterium has been observed. Berdy (CRC Handbook of Antibiotic Compounds, Vols. I-XIV, CRC Press, Inc., Boca Raton, Fla., 1980-1987) reports production of such antibiotics as ansamitocin-PDM-O, bacimethrin, megacin, pentapeptide, homopeptides. These are proteide antibiotics having relatively low mammalian toxicity. Additionally, B. megaterium was reported to inhibit three fungal pathogens of the rice phylloplane with the active fungicidal component being an antibiotic with a lipopeptide and polyoxin nature [Islam, K. Z., et al. (1985) Z. Pflkrankh und Pflschutz 92:233-240].
A variety of other uses for B. megaterium and its metabolites have been reported, including oxidation of selenium, phosphate solubilization, production of vitamin B.sub.12 in corn meal and the ability to degrade the herbicide metolachlor. The use of any B. megaterium strains to inhibit fungal growth, disease or infection in soybeans has not been described or reported.
In laboratory culture studies, one B. megaterium strain was found to oxidize selenium to selenite and a trace of selenate. This may be an important means of providing sufficient selenium to herbage to ultimately prevent selenium deficiency in animals [Sarathchandra, S. U., et al. (1981) Science 211:600-601]. A product, Phosphabacterin, from the U.S.S.R. and containing cells of B. megaterium var. phosphaticum, was used in the Soviet Union and Eastern Europe for the bacterization of crops to exploit the phosphate solubilization properties of the bacterium. Additional studies using the phosphate-solubilizing microorganisms on various crops in field studies were reported in India [Subba-Rao, N. S., (1982) in Advances in Agricultural Microbiology (N. S. Subbao-Rao, ed.) p. 219-242, Oxford Press, New Delhi: Oxford]. Various B. megaterium strains or isolates have been reported to produce vitamin B.sub.12 in corn meal [Chung, H. J. et al. (1986) J. Food Sci. 51:1514-1517] or to degrade metolachlor [Saxena, A. et al. (1989) Appl. Environ. Microbiol. 53:340-396] .